A focused ion beam system (FIB) is used for sample processing in a nano-range having several hundreds or less of nanometer in a sample. In particular, a sample for a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) is required to be processed so that the sample has a thin film shape having a substantially uniform thickness of several hundreds nanometers or less and including a nanoscale observation target position inside the sample. As processing of such a thin film sample, the thin film sample is prepared by milling a front surface and a back surface of the sample with a focused ion beam. Whether or not the observation target position is included in the sample is determined by observing a state of the front surface of the sample with the use of a scanning ion microscope image (Scanning Ion Microscope: SIM image) obtained by detecting secondary ions or secondary electrons induced by a focused ion beam with which the front surface of the sample has been irradiated and imaging the secondary ions or secondary electrons. In the case of the sample including the nanoscale observation target position, resolution of the SIM image is insufficient to observe a state of the front surface of the sample in some cases. For processing of such a sample, a FIB-SEM combined device (FIB-SEM) in which a scanning electron microscope (SEM) is combined with a FIB is used, and, a method of observing a surface of the sample processed by the FIB with the use of a high-resolution SEM image is used.
For observation using the TEM or STEM, it is essential that a desired nanoscale observation target position be included in a prepared thin film sample. For that purpose, it is important to stop milling using the FIB on both a front surface side and a back surface side of the sample at appropriate end points. As a method of obtaining end points, for example, PTL 1 discloses a method of setting points of time at which a sample has a film thickness determined in advance as end points on a front surface side and a back surface side thereof by providing, in a FIB-SEM, a detector for detecting electrons that have been transmitted through the sample and calculating a film thickness of the sample with the use of a contrast of a transmission electron image based on signals detected by the detector for detecting the transmitted electrons.
NPL 1 discloses that a Kikuchi pattern (back-scattered electron diffraction image) of an electron diffraction wave emitted from a back surface of a sample, the electron diffraction wave being caused by electrons injected through a front surface of the sample, is obtained.